1. Field of the Invention
The present invention relates to analog amplifiers for preamplifying low-level signals, and in particular, to analog preamplifier circuits having controllable dynamic signal ranges for amplifying charge-biased signals.
2. Description of the Related Art
High dynamic signal range is a key parameter for many types of circuits. This is particularly true in the area of flat panel X-ray imaging systems. As is well known in the art, such systems use a detector cassette containing a scintillation layer that absorbs and converts impinging X-ray photons to visible light photons for detection by photosensitive elements that are also within the detector array. As is further well known, such a detector array contains a two dimensional array of microscopic squares referred to as picture elements, or xe2x80x9cpixelsxe2x80x9d. Each pixel includes an addressable photosensitive element, such as a photodiode and switching transistor combination. From such circuitry individual pixel data signals, generally in the form of charge-based signals, are provided for amplification and further processing. Further discussion of this type of imaging system can be found in commonly assigned U.S. Pat. No. 5,970,115, entitled xe2x80x9cMultiple Mode Digital X-Ray Imaging Systemxe2x80x9d, the disclosure of which is incorporated herein by reference.
As part of the processing of such data signals, following preamplification and some form of a sample and hold operation, such signals are converted to digital signals using an analog-to-digital conversion circuit (ADC). Generally it is this ADC circuitry that sets, or limits, the maximum dynamic range of the system, typically at 14 bits. Such a maximum dynamic range, however, in the field of flat panel X-ray imaging systems has been an impediment to the commercial success of such systems. Accordingly, it would be desirable to have a technique whereby the maximum dynamic range can be extended and thus, be more independent from the maximum range of the ADC circuitry.
In accordance with the presently claimed invention, a dynamically controllable dynamic signal range is provided in the preamplifier stage of a data signal processing circuit that includes a downstream analog-to-digital signal converter. By monitoring the level of the data signal subsequent to its preamplification, the gain of the preamplifier stage is dynamically adjusted, thereby extending the effective, or usable, dynamic range of the preamplifier stage. This provides the further advantage of effectively extending the dynamic range of the overall system beyond that to which it would have otherwise been limited by the dynamic range of the analog-to-digital signal converter. One particularly advantageous application for this invention is in an X-ray imaging system such as that using flat panel solid state imaging devices.
Data signal amplification and processing circuitry with a dynamically controllable dynamic signal range in accordance with one embodiment of the presently claimed invention includes input and output terminals, amplification and processing circuitry, and control circuitry. The input terminal is for conveying an input data signal having a data signal charge associated therewith. The output terminal is for conveying an output data signal corresponding to the input data signal. The amplification and processing circuitry, coupled between the input and output terminals and including preamplification circuitry with a variable feedback capacitance associated therewith, receives a gain control signal and the input data signal and generates the output data signal, wherein a ratio of the output and input data signals is a function of the data signal charge and feedback capacitance and is responsive to the gain control signal. The control circuitry, coupled between the output terminal and the amplification and processing circuitry, monitors the output data signal and controls the variable feedback capacitance via the gain control signal, wherein the variable feedback capacitance is changed when the output data signal transcends a predetermined signal threshold.
An X-ray imaging system in accordance with one embodiment of the presently claimed invention includes an X-ray imaging device, amplification and processing circuitry, and control circuitry. The X-ray imaging device provides a plurality of pixel data signals having respective data signal charges associated therewith. The amplification and processing circuitry, coupled to the X-ray imaging device and including preamplification circuitry with a plurality of variable feedback capacitances associated therewith, receives one or more gain control signals and the plurality of pixel data signals and generates a plurality of output data signals corresponding respectively to the plurality of pixel data signals, wherein respective ratios of respective ones of the pluralities of output and pixel data signals are functions of corresponding respective ones of the data signal charges and feedback capacitances and are responsive to respective ones of the one or more gain control signals. The control circuitry, coupled to the amplification and processing circuitry, monitors the plurality of output data signals and controls the plurality of variable feedback capacitances via the one or more gain control signals, wherein respective ones of the plurality of variable feedback capacitances are selectively changed when one or more of the plurality of output data signals transcend one or more predetermined signal thresholds.